Hydrogels are hydrophilic polymer networks produced from reactions of one or more monomers or by association bonds between chains that can absorb from at least 20% to up to thousands of times their dry weight in water. Hydrogels may be chemically stable or they may disintegrate and dissolve with time. Hydrogels may be classified as either physical or chemical. Physical hydrogels have networks held together by molecular entanglements and/or secondary forces such as hydrogen bonding, van der Waals interactions, ionic or hydrophobic forces. Physical hydrogels are not homogeneous due to regions of high crosslinking density and low water swelling, called clusters, dispersed within low crosslinking density and high water swelling, or hydrophobic or ionic domains that create inhomogeneities. Chemical hydrogels are covalently crosslinked networks, but they may also be generated by crosslinking of water-soluble polymers, or by converting hydrophobic polymers to hydrophilic polymers. Chemical hydrogels are also not homogeneous due to clusters of molecular entanglements. Chain loops and free chain ends also produce network defects in both physical and chemical hydrogels, and they do not contribute to the permanent network elasticity.
An important characteristic of hydrogels is their swelling behaviour in water, since after preparation they have to be in contact with water to yield the final solvated network structure. Poly(vinyl alcohol) (PVA) is a hydrophilic polymer with various characteristics desired for biomedical applications, such as high degree of swelling, uncomplicated chemical structure, rubbery/elastic nature, and non-toxicity.
PVA has a relatively simple chemical formula with a pendant hydroxyl group and a crystalline nature, which allows it to form a solid hydrogel by the crosslinking of the PVA polymer chains. Vinyl alcohol (monomer) does not exist in a stable form and rearranges to its tautomer, acetaldehyde. PVA is produced by free radical polymerization of vinyl acetate to poly(vinyl acetate) (PVAc), and subsequent hydrolysis of PVAc gives PVA.
PVA can be crosslinked using several methods, such as the use of crosslinking chemical agents, using an electron beam or y-irradiation, or the physical crosslinking due to crystallite formation. For biomedical applications, physical crosslinking has the advantage of not leaving residual amounts of the toxic crosslinking agent, and also provides a hydrogel with higher mechanical strength than those obtained by crosslinking PVA using either chemical or irradiative techniques. In chemical cross-linking, chemical agents that can react with the hydroxyl groups are, for example, glutaraldehyde, ethylaldehyde, terephthalaldehyde, formaldehyde, hydrochloric, boric or maleic acid. Physical crosslinking forms a hydrogel with a network of semi-crystallites of hydrogen bonds filled with solvent.
To date, known methods of producing poly(vinyl alcohol) (PVA) and PVA hydrogel composites provide materials exhibiting the normal characteristic of isotropic mechanical behavior, that is, the mechanical properties of the material are the same regardless of orientation. This is expected due to the random distribution of the polymer chains. The typical tensile behavior for 10% PVA can be seen in FIG. 1, where the mechanical behavior is independent of sample orientation (isotropy).
Most tissues, including cardiovascular tissues, are composite viscoelastic biomaterials displaying mechanical properties with varying degrees of orientation effects. This orientation effect is due to the organization of the structural protein components such as collagen and elastin within the tissue. This organization gives rise to the unique exponential stress-strain relationship exhibited by soft tissues. Up to the present, PVA prepared under specific conditions has displayed some similar mechanical properties to certain soft tissues. However, there is no known synthetic biomaterial that displays anisotropic mechanical behaviour similar to soft tissue.
Even though there are several FDA approved materials for replacement aorta, such as Dacron or e-PTFE, these materials do not posses the same tensile properties as the tissue they are replacing, which results in hemodynamic problems and mismatch of mechanical properties and other problems at the implant/tissue junction.
Therefore, it would be very advantageous to be able to produce a material that displays mechanical properties that are similar to the tissue to be replaced. This would be an important step towards the development of, for example, cardiovascular devices with improved performance and durability.